1. Field of the Invention
The present invention relates to a thermal printer for printing a full-color image on a thermosensitive color recording material in which a plurality of coloring layers is formed, while the thermosensitive recording sheet is fed once in a direction.
2. Description Related to the Prior Art
In a color thermal printer, a full-color image is printed on a color thermosensitive recording sheet having at least three thermosensitive coloring layers. Thereby a thermal head heats the thermosensitive recording sheet to make coloring while the color thermosensitive recording sheet is shifted relatively to the thermal head. There is formed on a support a type of the color thermosensitive recording sheet having a cyan, a mazenta and a yellow coloring layers as the thermosensitive coloring layers. These three coloring layers have different thermosensitivities such that the coloring of them may be selectively made. The yellow coloring layer is disposed uppermost and has the largest thermosensitivity, and the cyan coloring layer is disposed at the lowest position on the support and has the smallest thermosensitivity. After recording a monochromatic image in one of the coloring layers, fixing of color is carried out such that a ultra-violet ray is illuminated on the one coloring layer in order to prevent the recorded coloring layer from coloring again when next coloring layer is recorded.
As the thermal printer, one head-three pass type and three heads-one pass type are well known. In the one head-three pass type, the thermosensitive recording sheet is fed back and forth three times, and thereby yellow, mazenta and cyan images are sequentially recorded in the respective coloring layers. In the three heads-one pass type, a yellow thermal head, a mazenta thermal head and a cyan thermal head are arranged along a feed path of the thermosensitive recording sheet. Further, two lamps are respectively disposed between the yellow and mazenta coloring thermal heads and between the mazenta and cyan coloring thermal heads to illuminate a ultra-violet ray on the thermosensitive recording sheet.
During feeding the thermosensitive recording sheet in the feeding direction, the yellow thermal head records the yellow image in the yellow coloring layer. Thereafter, an ultra-violet ray is illuminated on the thermosensitive recording sheet to carry out the fixing of the yellow image. Then the mazenta thermal head records the mazenta image in the mazenta coloring layer with a higher thermal energy than the yellow thermal head. Thereafter, an ultraviolet ray is illuminated on the thermosensitive recording sheet to carry out the fixing of the mazenta image. Finally, the cyan thermal head records the cyan image in the cyan coloring layer with the highest thermal energy. As described above, the recording of the yellow, mazenta and cyan monochromatic images and the fixing of color are sequentially carried out to form a full-color image.
However, in the three heads-one pass type of the thermal printer, all of the thermal heads have a same head touching conditions, such as offset length between the thermal head and the platen roller, and a withdraw angle at which the thermosensitive recording sheet is inclined for leaving from the thermal head, are same. Accordingly, there is a difference in the graininess between the monochromatic images recorded in the respective coloring layers. Further, there may be sometimes unevenness in glossy surface while a lubricant agent and the like are issued out of the thermosensitive recording sheet and are adhered through the thermal head on the thermosensitive recording sheet again. Thus, the quality of a print becomes lower.
A main object of the present invention is to provide a thermal printer for printing a full-color image of a good quality.
Another object of the present invention is to provide a thermal printer in which a lubricant agent does not adhered on a color thermosensitive recording material such that there may be no unevenness in glossy surface.
Still another object of the present invention is to provide a thermal printer with which each monochromatic image is recorded with an adequate graininess.
In order to achieve the above objects, in the thermal printer of the present invention, relative positions of the thermal heads to the confronting platen rollers along a feed direction are different corresponding to the respective thermosensitive coloring layers to be recorded. Namely, an offset length has different values corresponding to the thermal heads, while the offset length is defined as a length between an imaginary vertical line TCL passing a center of the heating elements and an imaginary vertical line PCL passing a rotational center of the platen roller.
When the TCL is in an upstream side of the feeding direction of the thermosensitive recording material from the PCL, the offset length has a positive sign, and when the TCL is in a downstream side of the feeding direction of the thermosensitive coloring material from the PCL, the offset length has a negative sign. When the uppermost layer, for example yellow thermosensitive coloring layer, is heated, the offset length is larger than when the lowest layer, for example a cyan coloring layer, is heated. In a preferable embodiment of the present invention, the offset length for the yellow thermosensitive coloring layer is +100 xcexcm, and that for the cyan thermosensitive coloring layer is xe2x88x92100 xcexcm.
Further, withdraw angles from the respective thermal heads are different corresponding to the thermosensitive coloring layers to be recorded. The withdraw angle is larger for recording in the cyan thermosensitive coloring layer than that in the yellow thermosensitive coloring layer.
According to the thermal printer of the present invention, monochromatic images are recorded in the respective recording layers to have the most adequate graininess. Further, a lubricant agent, even if it is issued out from a protective layer of the thermosensitive recording material, is not adhered through the thermal head to the thermosensitive recording material again. Therefore, there is no unevenness in glossy surface. The present invention achieves that the full-color image of high quality is printed on the thermosensitive recording material.